The Command and Control System

The command and control system of the balloon borne SBI is directly derived form the system used for FGE. Actually, several components (like the MAX1, microcontrollers, and the GPS) are the same used for FGE.

There are three main computers on-board: the Command and Control Computer (CCC), the Digital Acquisition Computer (DAC), and the actuator control computer (that we historically call MAX3). All computers use a commercial ATX mother board with a 1GHz Pentium III. MAX3 is the newest upgrade of the computer system. Up until SBI-1 MAX3 was a special-purpose board designed around a Dallas 87C520. After four flights and two winter overs in Antarctica this board showed signs of age and we decided to upgrade it to a more modern system (see below).

The CCC runs two separate processes: the Autonomous Control Executive (ACE) process and the Instrument Control (IC) process. The ACE is responsible for properly scheduling the operations performed by the gondola and to carry on the observational program. It can either operate totally autonomously or execute commands received directly from a ground control station via either UHF radio link (when the payload is in line-of-sight of the ground station) or via satellite relay through TDRSS.
The IC's main task is to provide a uniform interface for the ACE to a series of instrument subsystems. It also handles all the communications: it collects and transmits the housekeeping data and the I/O with all the instrument controllers.

The DAC controls the SBI detector, and is responsible for handling the stream of images coming from the frame grabber. It can transfer the image data to one of the 14 100GB hard drives (of high shock type), and it can perform simple data manipulations if needed: like averaging, subtracting, multiplying, or dividing frames.
The 14 hard drives are housed in a pressurized vessel and are liked to the DAC computer via USB2 to allow high data transfer rates. The DAC communicates directly to the CCC via an Ethernet link. It can handle commands arriving directly from the ACE process and can deliver images to the ACE for example to perform tasks such as auto-focus, pointing calibration or for downlinking images to the ground.

The IC process and the CCC interfaces with two instrument controllers, MAX1 and MAX3, the GPS, and CSBF's Support Instrument Package (SIP).
Max1 is a Motorola 68HC11E2 microprocessor. It is connected to the CCC via RS232 and handles the secondary focus actuator and the offset pointing motors.

MAX3 is a computer with a Pentium III processor. It collects a large fraction of the housekeeping data from the gondola, including temperatures, pressures, currents, and voltages. It also supplies the control voltages for the servo amplifiers that drive the three torque motors (elevation, reaction wheel, and momentum dump), and the discretes that switch such items as the stow latch. The most critical function of MAX3 is the pointing and control system. In this, MAX3 combines input data from the various pointing sensors to determine an "error," and from it and the current state of the payload, produces an output for either the elevation or reaction wheel drives to compensate and keep the SBI telescope steadily pointed at the Sun.

The CCC, DAC, MAX3, the hard drives, and other microprocessors are all commercial electronic products, thus not specifically designed to operate in a vacuum environment. They are all housed inside four pressurized vessels that maintain a stable pressure of 1 atm.